Many optical attenuators are known in the art. Some of these, use polarizing beam-splitters also known in the art, each having disadvantages. Furthermore, attenuators utilizing beam splitting to split an incoming beam into two polarized beams, have required one beam splitter to split an incoming beam into two orthogonally polarized beams, and one substantially identical beam splitter to combine the polarized beams into a single beam of light. Providing a matched pair of crystals can be challenging and is costly.
The Glan-Thompson polarizer, which is a block of birefringent material cut into prisms and then cemented together acts by reflecting one polarization component at the cement interface and by transmitting the other. The device requires a considerable amount of birefringent material, generally calcite, which is scarce and expensive, and is unable to work with high powered lasers and ultraviolet light, since the light destroys or clouds cement. Furthermore, this beam-splitter, which makes use of the reflected polarization component, suffers from the added disadvantage that polarized beams exit the device at inconvenient angles, for example 45 degrees, when it is often useful that beams are parallel, orthogonal or otherwise oriented.
The Glan-Taylor polarizer which is similar to the Glan-Thompson polarizer but uses an air space instead of cement to separate polarization components can work with many light sources but suffers from reflection loss and ghosting caused by the air gap.
The Wollaston, Rochon and Senarmont beam-splitters, which separates polarization components by transmitting the components through an interface, permit optical contacting for use with most light sources, but produce beams which also exit at inconvenient angles, with one or both polarization components suffering from chromatism and distortion.
The double refracting element (beam displacer), which produces parallel polarized beams of light, achieves small beam separation and limited field. Also, since the beams may pass through a considerable amount of material before achieving useful separation, wavefront distortion can occur in the extraordinary beam due to imperfections in the crystal's structure. (See for example, "Birefringence of Quartz and Calcite", Journal of the Optical Society of America, volume 49, No. 7, Jul. 1959, pages 710-712.) Beam separation can be further limited by the small size and high cost of suitable crystals. Notwithstanding, it is an object of the invention to overcome some of these limitations while using a beam displacing crystal. Polarizing prisms and their various defects are described in detail by Bennett and Bennett, "Polarization", Handbook of Optics, Driscoll and Vaughan, eds., McGraw-Hill, 1978.
U.S. Pat. No. 5,727,109 incorporated herein by reference, in the name of E-tek Dynamics discloses an Optical Attenuator having two lenses, two crystals and a liquid crystal cell for varying the polarization of optical beams passing therethrough. Although this device may perform its intended function, there are several drawbacks to this design. For example, it requires the provision of two substantially thick optical crystals to accommodate a collimated beam provided by a collimating lens. Since these crystals must be large in order to accommodate and separate a collimated beam, their cost is quite significant. Furthermore, This device requires a pair of crystals, yet further adding to the cost of the device. And still further, care must be taken to provide substantially matched crystals, adding further to the cost of manufacture of this device. And yet another drawback of this design is its size, which is larger than would be preferred by some.
The present invention obviates most or all of these disadvantages and results in a small relatively inexpensive optical attenuator.
It is an object of this invention, to provide, in a first embodiment a polarization beam splitter that obviates most of the above mentioned disadvantages of prior art devices, and an optical attenuator that is less complex and less costly to manufacture; it is yet a further object to provide in one embodiment, a device having as its core element a birefringent crystal that is considerably less costly to manufacture than commercially available birefringent crystal based devices.
It is yet a further object of the invention to provide a polarization beam splitter that requires a much smaller crystal than conventional commercially available devices that utilized a same type of beam shifting crystal.
It is a primary object of this invention to provide an optical attenuator that only requires a single splitting/combining optical element for splitting an optical beam incident thereon, into two substantially orthogonally polarized beams and for combining the beams into a single beam.
It is yet a further object of the invention to provide an optical attenuator that has a reflective folded configuration.